A. Field of the Invention
The present invention relates to a deflection yoke for a CRT (cathode ray tube), including a device for correcting misconvergence and upper and lower raster distortions of the electron beams, thereby forming images of high quality.
B. Description of the Prior Art
A conventional CRT, as shown in FIG. 1, comprises a panel 2 having an RGB fluorescent film 1 on the inner surface, a funnel 3 fused to the rear end of the panel 2, electron guns 5 sealed in the neck 4 of the fumnel 3, a deflection yoke 6 for deflecting electron beams generated from the electron guns 5 onto the whole surface of the fluorescent film 1, and a shadow mask 7 installed in the panel 2 and having a plurality of holes so as for the deflected electron beams to pass through.
The deflection yoke 6 is composed of a horizontal deflection coil 8 for horizontally deflecting the electron beams generated from the electron guns 5, a vertical deflection coil 9 for vertically deflecting the electron beams, a conical ferrite core 10 to enhance the magnetic efficiency by reducing the loss of magnetic force generated from the horizontal and vertical deflection coils 8 and 9, and a holder 11 fixing horizontal and vertical deflection coils 8 and 9 and ferrite core 10 at defined positions and isolating the horizontal deflection coil from the vertical one 9.
As shown in FIG. 6, the horizontal coil 8 consists of a main coil 81, and an auxiliary coil 12 that is independently formed between the middle and opening portions of the main coil 81. As shown in FIG. 2, the auxiliary coil 12 is positioned between the middle and opening portions on the inner surface of the holder 11, generating a local horizontal defection magnetic field.
Below describes the operation of the conventional deflection yoke as constructed above.
When the CRT is activated with power, electron guns (not shown) generate electron beams, which are deflected by the deflection yoke 6.
As mounted on the inner surface of the holder 11 and affected by the horizontal deflection magnetic field, the auxiliary coil 12 generates the magnetic field which forms a second magnetic field in such a direction that would oppose the horizontal deflection magnetic field generated by the main coil 81 of the horizontal deflection coil 8 according to the Lenz's law (the magnetic field induced by the current is in a direction that the current it would produce compensates for the change which causes the induced magnetic field). Consequently, the whole horizontal deflection magnetic field a' forms a local, horizontal deflection pin magnetic field b', as shown in FIG. 3.
Where convergence errors occur as shown in (a) of FIG. 4, the horizontal deflection pin magnetic field compensates for the convergence errors of the horizontal, red and blue beams R and B, so that the red and blue beams correspond with each other as shown in (b) of FIG. 4.
With upper and lower raster distortions errors as shown in (a) of FIG. 5, the horizontal deflection pin magnetic field compensates for the upper and lower distortion errors on the screen to be correspondence with each other as shown in (b) of FIG. 5.
Such a conventional deflection yoke for CRTs has an auxiliary coil that is made by winding a copper wire of the same type as the main coil of the horizontal deflection coil with a desired number of turns by use of an auxiliary coil-winding die, applying currents to both ends of the coil so as to melt the adhesive layer deposited on the surface of the copper coil, forming a desired shape of the auxiliary coil, and making a closed circuit by peeling the coating off the auxiliary coil at both ends and connecting both naked ends by a soldering step. The complexity of this process for making an auxiliary coil increases the number of processes and also requires a separate winding machine, with a consequence of an increase in production cost for the auxiliary coil and profit reduction of companies.
While the auxiliary coil is formed with uniform internal profile by the aid of a winding die the external profile of the auxiliary coil may be adversely affected by the state of winding. This affects the deflection magnetic field generated from the auxiliary coil for its intensity and profile, which leads to misconvergence and deviation of raster distortion on the screen.
In an installation the auxiliary coil is too inflexible to mount in accordance with the curvature of the inner surface of the holder, which also causes the above problems with the misconvergence and deviation of raster distortion on the screen.